The present invention relates to rotary fluid pressure devices, and more particularly, to such devices which are used in closed loop hydraulic circuits, wherein the rotary fluid pressure device includes a shuttle valve arrangement.
Although it should become apparent from the subsequent description that the invention may be useful with many types of rotary fluid pressure devices, including both pumps and motors, it is especially advantageous when used with a low speed, high torque hydraulic motor, and will be described in connection therewith. Furthermore, although the invention may be used with fluid pressure devices having various types of displacement mechanisms, the invention is especially useful in a device including a gerotor displacement mechanism, and will be described in connection therewith.
The use of low speed, high torque gerotor motors is becoming increasingly common in closed loop hydraulic circuits, i.e., a circuit in which the outlet port of the motor is connected directly to the inlet port of the pump, rather than to the system reservoir. This is especially true in regard to mobile applications, such as construction equipment in which hydraulic motors are used to drive the vehicle wheels.
In closed loop circuits of the type described, it is frequently necessary to divert a portion of the return fluid flow, from the motor to the pump, and pass it through a heat exchanger to prevent overheating of the system fluid. This is normally accomplished by means of a shuttle valve assembly installed in the motor to provide fluid communication between the low pressure side of the motor and a shuttle port. The shuttle port is then connected by means of a cooler line to the inlet of a heat exchanger, and after passing through the heat exchanger, this diverted fluid flows to the pump inlet.
One of the problems associated with hydraulic motors having shuttle valves has been a condition referred to as "hunting" of the shuttle valve. This typically occurs when the motor is operating at a relatively low pressure differential which causes the shuttle valve to become unstable. The low pressure differential can cause the shuttle piston to oscillate (or "hunt") rapidly, causing a clicking noise which may be mistakenly interpreted by the vehicle operator as a malfunction in the drive system associated with the motor. The oscillation of the shuttle valve may also result in unnecessary fatigue of the shuttle valve parts.